Lithostratigraphic development and neotectonic significance of the Quaternary sediments along the Kachchh Mainland Fault (KMF) zone,western India

The Kachchh Mainland Fault (KMF) is a major E–W trending seismically active fault of the Kachchh palaeorift basin whose neotectonic evolution is not known. The present study deals with the eastern part of the KMF zone where the fault is morphologically expressed as steep north facing scarps and is divisible into five morphotectonic segments. The Quaternary sediments occurring in a narrow zone between the E–W trending KMF scarps and the flat Banni plain to the north are documented. The sediments show considerable heterogeneity vertically as well as laterally along the KMF zone. (The Quaternary sediments for a northward sloping and are exposed along the north flowing streams which also show rapid decrease in the depth of incision in the same direction.) The deposits, in general, comprise coarse as well as finer gravelly deposits, sands and aeolian and fluvial miliolites. The Quaternary sediments of the KMF zone show three major aggradation phases. The oldest phase includes the colluvio-fluvial sediments occurring below the miliolites. These deposits are strikingly coarse grained and show poor sorting and large angular clasts of Mesozoic rocks. The sedimentary characteristics indicate deposition, dominantly by debris flows and sediment gravity flows, as small coalescing alluvial fans in front of the scarps. These deposits suggest pre-miliolite neotectonic activity along the KMF. The second aggradation phase comprises aeolian miliolites and fluvially reworked miliolites that have been previously dated from middle to late Pleistocene. The youngest phase is the post-miliolite phase that includes all deposits younger than miliolite. These are represented by comparatively finer sandy gravels, gravelly sands and sand. The sediment characteristics suggest deposition in shallow braided stream channels under reduced level of neotectonic activity along the KMF during post-miliolite time evidenced by vertical dips of miliolites and tilting of gravels near the scarps. The tectonically controlled incision and dissection of the Quaternary deposits is the result of neotectonic activity that continues at present day. The overall nature, sedimentary characteristics and geomorphic setting of the sediments suggest that the KMF remained neotectonically active throughout the Quaternary period.     

The Hunghae Formation, SE Korea: Miocene debris aprons in a back-arc intraslope basin

ABSTRACT During early to middle Miocene times a sudden opening of the Ulleung (Tsushima) back-arc basin in the East Sea (Sea of Japan) led to the development of intraslope basins along the rifted southwestern margin (southeast Korea). Abrupt subsidence resulted in the deposition of the 200 m thick Hunghae Formation (middle Miocene), a sand/mudstone sequence that can be divided into five facies. Facies I (sand and mudstone couplet) and II (coarse sand) are turbiditic in origin, as evidenced by massive, graded, crudely-layered and parallel-laminated sand beds. Facies III (homogeneous mudstone) is characterized by various lignite and plant fragments, clastic and biogenic grains that are randomly oriented, suggestive of hemipelagic deposition. Facies IV (chaotic deposit) is characterized by the disruption of beds, the presence of isolated siltstone blocks (or balls) and large clasts in the muddy matrix, indicative of retrogressive rockfall and slide/slump. Facies V (conglomerate) is of debris flow origin, as evidenced by clast- and matrix-supported features, floating large clasts and absence of traction structures. Individual facies are organized into two types of facies association: (1) homogeneous mudstone (facies III) associated randomly with the rest (facies I, II, IV and V), indicative of hemipelagic and episodic sediment-gravity flow processes, respectively; (2) conglomerate (facies V), coarse sand (facies II) and sand/mudstone couplet (facies I), representing the flow transformation from debris flow to high- and low-concentration turbidity currents. These facies associations are similar in many respects to modern and ancient debris (or slope) aprons found elsewhere. Numerous isolated slide/slump blocks, wedged conglomerates with armoured mudstone balls, discontinuous lignite-containing sand/mudstone beds, chaotic structure and growth faults suggest that the deposition occurred on a steep slope (intraslope basin) off coalescing fan-deltas, mainly by unchannellized sediment-gravity flows. Ancient deposits with irregular facies sequences can be viewed as debris-apron systems, which provide alternatives to submarine-fan models in many clastic basins with a line rather than point source.     

辽中凹陷旅大16油田东三段层序地层及沉积充填

渤海湾盆地辽中凹陷旅大16油田东营组东三段是目前勘探开发的主力含油层系。综合三维地震资料、录井、测井及岩心资料,在井—震结合、层序地层学研究基础上,利用古地貌研究、岩心相、测井相、地震相和振幅属性切片分析,明确了东营组层序格架内沉积相类型、沉积展布特征及沉积演化规律。应用Vail的经典层序地层学理论,将东三段划分为一个三级层序,其中可识别出低位体系域、湖侵体系域和高位体系域。低位体系域沉积期,辽西低凸起大面积暴露遭受剥蚀,为研究区提供碎屑沉积物,发育近源的扇三角洲及重力流沉积体系。扇三角洲砂体以灰色中砂岩和细砂岩为主,河床滞留沉积、波状层理和生物扰动构造十分发育;高位体系域时期,辽西低凸起被淹没,供源能力减弱,研究区发育较远源的辫状河三角洲前缘沉积,且伴生重力流沉积。其中辫状河三角洲前缘水下分流河道砂体以细—中砂岩为主,见块状构造和楔状交错层理。重力流沉积体系主要为滑塌湖底扇及砂质碎屑流沉积,岩心观察可见细砂岩中发育滑塌变形构造,及漂浮状的砾石和泥岩撕裂屑。研究区沉积演化过程与地貌演化密切相关,且断裂体系对砂体展布具有控制作用,重力流沉积发育于断裂坡折前方。显然,明确沉积相类型、沉积展布特征及沉积演化规律,可为进一步油气勘探开发提供地质依据。     

Depositional features of a late Weichselian outwash fan; central East Jylland,Denmark

Four major sedimentary facies are present in coarse-grained, ice-marginal deposits from central East Jylland, Denmark. Facies A and B are matrix-supported gravels deposited by subaerial sediment gravity flows as mudflows (facies A) and debris flows (facies B). Facies C consists of clast-supported, water-laid gravels and facies D are cross-bedded sand and granules. The facies can be grouped into three facies associations related to the supraglacial and proglacial environments: (1) the flow-till association is made up of alternating beds of remobilized glacial mixton (facies A) and well-sorted cross-bedded sand (facies D); (2) the outwash apron association resembles the sediments of alluvial fans in containing coarse-grained debris-flow deposits (facies B), water-laid gravel deposited by sheet floods (facies C) and cross-bedded sand and granules (facies D) from braided distributaries; (3) the valley sandur association comprises water-laid gravel (facies C) interpreted as sheet bars and longitudinal bars interbedded with cross-bedded sand and granules (facies D) deposited in channels between bars in a braided environment.The general coarsening-upward trend of the sedimentary sequences caused by the transition of bars and channel-dominated facies to debris-flow-dominated facies indicate an increasing proximality of the outwash deposits, picturing the advance and still stand of a large continental lowland ice-sheet. The depositional properties suggest that sedimentation was caused by melting along a relatively steep, active glacier margin as a first step towards the final vanishing of the Late Weichselian icesheet (the East Jylland ice) covering eastern Denmark.     

鄂尔多斯盆地华池地区长6油层组湖底扇内深水重力流沉积特征

以岩心的详细观察与测井录井资料综合分析为基础,通过岩石学特征和原生沉积构造等沉积相标志分析,确定鄂尔多斯盆地华池地区长6油层组厚层块状砂体属于半深湖内的深水重力流沉积体系。划分出内扇、中扇和外扇3个亚相,该区主要以中扇亚相最为发育;在深水重力流中识别出液化沉积物流、砂质碎屑流、近源浊流和远源浊流等沉积类型,建立了湖盆深水重力流沉积模式。砂质碎屑流沉积最为广泛,砂体厚度大,连片发育,具有最好的储集层物性,含油性好,是华池地区长6油层组最有利的储集层。     

Pleistocene glaciolacustrine sedimentation in a tectonically active zone, Kleszczów Graben, central Poland

The Kleszczów Graben in central Poland was formed by late Oligocene to Middle Pleistocene extensional tectonics. During the Pleistocene it was infilled with a 200 m thick sequence of predominantly glacial sediments. Four distinct formations of Elsterian and Saalian age are identified, each containing 15–40 m of glaciolacustrine strata. The boundaries between formations are marked by erosional surfaces and, in part, by angular discordances caused by tectonism. Glaciolacustrine sedimentation was tectonically controlled: the thickness of the sequences in the graben are three to five times greater than outside the area of fault-controlled subsidence. Deposition in the proglacial lakes was controlled by differential subsidence rates within the basin: deep-lake facies (varved clays) were deposited in sub-basins with high subsidence rates and deltaic to shallow-water facies accumulated in areas of moderate subsidence or occasional uplift. These variations led to the development of a very complex, ‘mosaic’ of lateral facies relationships, suggesting that several sub-basins with differing subsidence rates were present. The Vertical successions show proximal-distal sequences typical of glacier-fed lakes that have limited contact with the ice sheet. However, gravity flow facies are very common, and occur both in the shallow- and deep-water deposits. These deposits are interpreted to have been formed adjacent to active fault scarps which bordered the lake basin. Although several distinct phases of glaciolacustrine sedimentation occurred during the history of trough infilling, the location of the areas of high subsidence varied through time.     

Arcuate and growth faults in basin models, with an application to a south Louisiana roll-over structure

Using a two-dimensional fluid-flow/compaction code an investigation is given of the influence of dynamically evolving curved faults in impacting the overpressure development of a sedimentary basin. Four synthetic cases are examined: an homogeneous sandy-shale section, and a mixed layered section of sand and shale, with fault conditions investigated being open to fluid flow, and closed to fluid-flow, respectively. Fault throws can be instantaneous either in time (such as a gravity slump) or can be progressive in time (such as a growth fault). A case history from south Louisiana also is examined to illustrate typical response patterns.     

Syn‐rift mass flow generated ‘tectonofacies’ and ‘tectonosequences’ of the Kingston Peak Formation,Death Valley,California, and their bearing on supposed Neoproterozoic panglacial climates

The Kingston Peak Formation of the Pahrump Group in the Death Valley region of the Basin and Range Province, USA, is the thick (over 3 km) mixed siliciclastic–carbonate fill of a long‐lived structurally‐complex Neoproterozoic rift basin and is recognized by some as a key ‘climatostratigraphic’ succession recording panglacial Snowball Earth events. A facies analysis of the Kingston Peak Formation shows it to be largely composed of ‘tectonofacies’ which are subaqueous mass flow deposits recording cannibalization of older Pahrump carbonate strata exposed by local faulting. Facies include siltstone, sandstone and conglomerate turbidites, carbonate megabreccias (olistoliths) and related breccias, and interbedded debrites. Secondary facies are thin carbonates and pillowed basalts. Four distinct associations of tectonofacies (‘base‐of‐scarp’; FA1, ‘mid‐slope’; FA2, ‘base‐of‐slope’; FA3, and a ‘carbonate margin’ association; FA4) reflect the initiation and progradation of deep water clastic wedges at the foot of fault scarps. ‘Tectonosequences’ record episodes of fault reactivation resulting in substantial increases in accommodation space and water depths, the collapse of fault scarps and consequent downslope mass flow events. Carbonates of FA4 record the cessation of tectonic activity and resulting sediment starvation ending the growth of clastic wedges. Tectonosequences are nested within regionally‐extensive tectono‐stratigraphic units of earlier workers that are hundreds to thousands of metres in thickness, recording the long‐term evolution of the rifted Laurentian continental margin during the protracted breakup of Rodinia. Debrite facies of the Kingston Peak Formation are classically described as ice‐contact glacial deposits recording globally‐correlative panglacials but they result from partial to complete subaqueous mixing of fault‐generated coarse‐grained debris and fine‐grained distal sediment on a slope conditioned by tectonic activity. The sedimentology (tectonofacies) and stratigraphy (tectonosequences) of the Kingston Peak Formation reflect a fundamental control on local sedimentation in the basin by faulting and likely earthquake activity, not by any global glacial climate.     

辽河坳陷双台子构造带沙三段沉积相展布特征与沉积主控因素

辽河西部凹陷双台子构造带E₂s₃发育重力流沉积,沉积相类型为湖底扇中扇和外扇,同沉积断层和古地貌是控制沉积的主要因素。论文描述了同沉积断层和古地貌的展布特征,并以此为基础,开展了沉积相主控因素的研究工作。建立了“沟道控砂,坡度控形,物源控供给”的沉积模式:辫状沟道沉积分布在受同沉积断层和古地貌的联合控制的古沟道内。古地貌坡度小,辫状沟道沉积向盆地中心搬运的距离长,沟道弯曲度大,沉积分异作用更加明显,盆地内主要以砂岩与泥岩互层为特征（双南）。古地形坡度大,辫状沟道沉积向盆地中心搬运距离短,沟道弯曲度小,沉积分异作用差,盆地内主要以厚层砂岩和薄层泥岩组合为特征（双台子）。双台子和双南具有不同的物源,两个地区在同一层序内具有不同的砂体叠置样式。双南和双台子沉积作用机制上的不同,导致了二者在沉积相带展布、砂体叠置样式等方面的差异。对储集体而言,双台子比双南具有更有利的沉积相带。     

On steady homogeneous sand–water flows in a vertical conduit

In this paper, an idealized model of the steady‐state phase of the flow in a vertical conduit leading to a sand volcano eruption is developed from first principles. The model assumes that a sand–water mixture flows upwards, driven by an overpressure at the base of a vertical cylindrical conduit (or a two‐dimensional fracture) and opposed by gravity, viscous resistance and turbulent drag. The conditions for flow are analysed in detail, and the mechanisms controlling the eruption rates are studied quantitatively. The flow predictions are in accordance with our observations of analogous vigorous sand eruptions at deepwater oil fields. For sufficiently high flow velocities (u > 10u_T) and small sand fractions (s < 0·2), the flow may be well mixed and homogeneous. If these conditions are not met, the flow may either become two phase or does not develop. Combining geological considerations with the steady homogeneous model, it is possible to predict the behaviour of the vigorous quasi‐steady stage of a sand volcano eruption. It is shown that, based on the average density of the overlying sediments, there are a range of overpressures for which sand volcanoes may develop.     

'Linked' debrites in sand-rich turbidite systems – origin and significance

Abstract The outer parts of a number of small Late Jurassic sandy deep‐water fans in the northern North Sea are dominated by the stacked deposits of co‐genetic sandy and muddy gravity flows. Sharp‐based, structureless and dewatered sandstone beds are directly overlain by mudclast breccias that are often rich in terrestrial plant fragments and capped by thin laminated sandstones, pseudonodular siltstones and mudstones. The contacts between the clast‐rich breccias and the underlying sandstones are typically highly irregular with evidence for liquefaction and upward sand injection. The breccias contain fragments (up to metre scale) of exotic lithologies surrounded by a matrix that is extremely heterogeneous and strewn with multiphase and variably sheared sand injections and scattered coarse and very coarse sand grains (often coarser than in the immediately underlying sand bed). Markov chain analysis establishes that the breccias consistently overlie sandstones, and the character of the breccias and their external contacts rule out a post‐depositional origin via in situ liquefaction, intrastratal flowage or bed amalgamation and disruption. The breccias are interpreted as debrites that rode on a water‐rich sand bed just deposited by a co‐genetic concentrated gravity current. As such, they are referred to as ‘linked debrites’ to distinguish them from debrites emplaced in the absence of a precursor sand bed. The distinction is important, because these linked debris flows can achieve significant mobility through entrainment of both water and sediment from beneath, and they ride on a low‐friction carpet of liquefied sand. This explains the paradox presented by fan fringes in which there are common debrites, when conventional thinking might predict that deposits of low‐concentration gravity currents should be more important here. In fact, evidence for transport by low‐concentration turbidity currents is rare in these systems. Several possible mechanisms might explain the formation of linked flows, but the ultimate source of both sandy and clast‐rich flow components must be in shallower water on the basin margin (the debrites are not triggered from distal slopes). Flow partitioning may have occurred by upslope erosion and retardation of the mudclast‐charged portion of an erosional sandy density current, partial flow transformation of a precursor debris flow and/or hydraulic segregation and reconcentration of the flaky clasts and carbonaceous matter during transport. Linked debrites are not restricted to small sand‐rich fans, and similar mechanisms may be responsible for the long runout of debris flows in other systems. The recognition of a distinct class of linked debrites is of wider importance for facies prediction, reservoir heterogeneity and even carbon fluxes and sequestration on continental margins.     

Facies architecture of individual basin‐plain turbidites: Comparison with existing models and implications for flow processes

Current understanding of submarine sediment density flows is based heavily on their deposits, because such flows are notoriously difficult to monitor directly. However, it is rarely possible to trace the facies architecture of individual deposits over significant distances. Instead, bed‐scale facies models that infer the architecture of ‘typical’ deposits encapsulate current understanding of depositional processes and flow evolution. In this study, the distribution of facies in 12 individual beds has been documented along downstream transects over distances in excess of 100 km. These deposits were emplaced in relatively flat basin‐plain settings in the Miocene Marnoso Arenacea Formation, north‐east Italy and the late Quaternary Agadir Basin, offshore Morocco. Statistical analysis shows that the most common series of vertical facies transitions broadly resembles established facies models. However, mapping of individual beds shows that they commonly deviate from generalized models in several important ways that include: (i) the abundance of parallel laminated sand, suggesting deposition of this facies from both high‐density and low‐density turbidity current; (ii) three distinctly different types of grain‐size break, suggesting waxing flow, erosional hiatuses and bypass of silty sediment; (iii) the presence of mud‐rich debrites demonstrating hybrid flow deposition; and (iv) dune‐scale cross‐lamination in fine‐medium grained sandstones. Submarine sediment density flows in basin‐plain settings flow over relatively simple topography. Yet, their deposits record complex flow events, involving transformation between different flow types, rather than the simple waning surges often associated with the distal parts of turbidite systems.     

沾化凹陷孤岛西部斜坡带沙三段重力流沉积特征与源-汇体系

以岩芯观察、粒度分析、薄片鉴定、测井资料和地震资料解释等为主要手段,研究渤海湾盆地沾化凹陷孤岛西部斜坡带沙三段主要沉积物重力流类型及其沉积特征,探讨不同触发机制下的沉积物重力流演化过程和构造活动对重力流沉积过程及砂体展布的控制,总结源-汇耦合体系,建立斜坡带重力流砂体发育模式。结果表明:研究区沙三段沉积时期发育异重流、碎屑流、浊流、液化流和滑动-滑塌五种沉积物重力流,具有洪水型和滑塌型两种触发机制,流体演变总体处于碎屑流向浊流演化的早期阶段,推测研究区以北深水区仍发育碎屑流沉积且开始广泛发育浊流沉积。构造作用对研究区沙三段流体性质与演化、同生变形构造和重力流成因砂体的发育与分布具有明显的控制作用。总体上,研究区具有断槽沟谷、断裂坡折、断裂走向斜坡及缓坡沟谷等4种主要的源-汇耦合体系。纵向上,研究区沙三段自下而上由（半）深湖、近岸水下扇、滑塌扇沉积演变为滨浅湖、辫状河三角洲以及扇三角洲沉积;平面上,研究区东部主要发育来自孤岛凸起的扇三角洲前缘和近岸水下扇,西部主要发育来自陈家庄凸起的辫状河三角洲前缘,中部主要发育串珠状滑塌扇体。     

西秦岭直合隆地区三叠系深水水道沉积模式分析

西秦岭三叠系地层主要由深水沉积物组成。选取西秦岭直合隆地区出露较好的三组剖面进行研究,按重力流类型及静水条件下所发生的沉积现象,总结出六种岩石相类型,即滑塌岩相（F1）、碎屑流相（F2）、超高密度流相（F3）、高密度浊流相（F4）、低密度浊流相（F5）、深海泥岩相（F6）。通过对三处剖面的横向追踪对比,结合不同沉积环境下表现出特定的岩石相占比及砂体堆叠样式,共识别出限制性水道、弱限制性水道及水道天然堤三类沉积单元。限制性水道中砂体叠置关系复杂,以超高密度流相占主导地位,次为碎屑流相;弱限制性水道中砂体叠置较规整,以超高密度流相占主导地位,次为低密度浊流相;而水道天然堤表现为砂泥互层（总厚度约15 m）,主要为低密度浊流相和深海泥岩相。最后建立了研究区内深水水道沉积演化阶段模式图——早期限制性环境下水道较顺直,水道较窄,砂体叠置关系复杂;中期限制性环境相对早期有所减弱,水道弯曲度增大,水道变宽,出现溢岸沉积,砂体叠置关系复杂;晚期为弱限制性环境,弯曲水道两侧发育天然堤,砂体叠置规整。本次研究利用野外露头建立的深水水道沉积演化模式,在一定程度上还原其相关的重力流演化过程及发育期次,对全球其他类似研究具有一定的参考意义。     

The Maesan fan delta, Miocene Pohang Basin, SE Korea: architecture and depositional processes of a high-gradient fan-delta-fed slope system

The Maesan fan-delta-fed slope system in the Miocene Pohang Basin occurs between two Gilbert-type fan deltas. Detailed analysis of sedimentary facies and bed geometry reveals that the sequence is represented by 13 sedimentary facies. These facies can be organized into three facies associations, representing distinct depositional environments: alluvial fan (facies association I), steep-faced slope (facies association II), and basin plain (facies association III). Subaerial debris flows and dense, inertia-dominated currents were transformed into subaqueous sediment gravity flows in steep-faced slope environments. Further downslope, these flows were channelized and formed lobate conglomerate and sandstone bodies at the terminal edge of the channels (or chutes). Interchannel and interlobe areas were dominated by homogeneous mudstone and muddy sandstone, deposited by suspension settling of fine-grained materials. Part of the steep-faced slope deposits experienced large-scale slides and slumps. The chutes/channels, lobes and splays on the steep-faced slope of the Maesan system are similar to those in modern subaqueous coarse-grained fan-delta systems.     

陆相湖盆深水重力流沉积特征、砂体结构研究及油气勘探意义—以鄂尔多斯盆地上三叠统长7油层组为例

结合深水重力流沉积理论、野外剖面、岩心、测井等资料的分析,对鄂尔多斯盆地合水地区延长组长7油层组重力流沉积相和砂体结构的类型、特征进行了深入分析。研究表明长7沉积期发育大面积的深水砂岩,主要发育砂质碎屑流、浊流等重力流砂体,砂质碎屑流沉积以块状层理为主,单砂体厚度大,浊流沉积以粒序层理为主,砂泥互层频繁;结合重力流沉积背景下各成因砂体测井曲线形态、垂向发育厚度、垂向连续性及组合特征,将砂体垂向叠置结构划分为多期砂叠置厚层型、厚砂-薄泥互层型、厚砂-薄砂-泥互层型及薄砂-厚泥互层型四种类型,并对研究区单井砂体结构进行了聚类。实践表明,砂质碎屑流沉积区虽是油气藏有利聚集分布带,但分布面积较广,结合砂体结构特征,可进一步细化有利区划分,为致密油藏有利区的优选提供了新的研究思路。     

地震沉积学在识别重力流沉积体系中的应用

探讨地震沉积学方法在识别陆相断陷盆地重力流沉积体系的研究思路。以歧口凹陷歧南斜坡沙一段的深水厚层砂岩为研究对象, 在现代沉积学和地震沉积学理论和方法的指导下、分频解释和时频分析技术建立高频等时格架的基础上, 通过岩芯相精细描述确定沙一段主要发育重力流水道沉积体系。形成了以古地貌和地震属性约束物源方向, 预测重力流水道的宏观展布;地层切片、反演与三维可视化相结合刻画砂体空间展布形态的断陷湖盆沟道型重力流地震沉积学识别方法和技术。应用该方法对歧南斜坡重力流水道砂体进行了精细刻画, 为在歧口凹陷广泛发育的湖相泥岩中寻找岩性油气藏开辟新的勘探领域, 并在歧口凹陷油气勘探中取得了良好的应用效果, 同时也丰富和发展了断陷盆地重力流沉积体系的地震沉积学研究成果。     

陕西富平中-上奥陶统深水碳酸盐重力流沉积模式

陕西富平地区,在中-晚奥陶世位于华北地台南侧的弧后盆地的大陆边缘。其中完好地保存了一套以半远洋的泥晶灰岩夹重力流的角砾灰岩和砂屑灰岩为特征的深水沉积岩系。一般认为,与半远洋的石灰岩共生的碳酸盐重力流沉积的复杂岩相,是大陆坡的标志,而相应的陆源碎屑沉积,则被作为海底扇的产物。然而,富平地区的碳酸盐重力流沉积,却是在弧后盆地伸进浅水台地之间的深海前槽中,沿海槽轴向呈席状流搬运、沉积的。     

Umbrella structure and channel-wall stoping in the Cambrian St. Roch Formation,Quebec Appalachians: significance for particle support mechanisms and turbulence development in hyper-concentrated sediment gravity flows

Umbrella structure is a newly recognized sedimentary structure associated with large platy clasts in resedimented boulder-bearing pebble conglomerate with a sandy matrix. It consists of a sand rim that lacks pebbles on parts or the entire underside of platy boulders, whereas on the upper side, pebbles are in direct contact with the boulders. The depositing processes were high- to hyper-concentrated sediment gravity flows in a submarine channel or canyon on the Cambrian continental slope of North America bordering the Iapetus Ocean. The structure occurs predominantly where clasts dip moderately in the down-current direction. Based on the association of the structure with slightly forward dipping slabs, it is proposed that these down-current dipping slabs may have been in the process of counter-clockwise rotation that was aborted and may have generated a pressure shadow on the underside enabling the inrush of fluid and the infiltration of sand into the anomalous low-pressure zone. The structure has implications for particle support mechanisms in high- to hyper-concentrated sedimentary gravity flows, in that it redirects attention to the much debated mechanism of dispersive pressure and alternatives. It provides an observable sediment structure that supports dispersive pressure which so far depended on experimental evidence and theoretical arguments alone. Vrolijk and Southard’s (1997) concept of a ‘laminar sheared layer’ is here for the first time interpreted as having an upward-moving ‘free-surface’ layer effect during deposition from hyper-concentrated flows. Channel-wall stoping involves unlithified turbiditic spillover sand in the levee sediment of the canyon wall that was washed out by the upper diluted parts of the high-concentration flows coming down the channel and leaving a niche in the wall that was filled with coarser channel-axis facies by the same flow (or later flows) when its aggradation reached the level of the niche. The contact between turbidite and pebble conglomerate occurs now more than 2 m laterally into the exposed channel wall. Channel-wall stoping tracks turbulence development in hyper-concentrated gravity flows.     

Proglacial glaciotectonic deformation associated with glaciolacustrine sedimentation,Lake Pukaki,New Zealand

The Quaternary glaciogenic sediments exposed on the southwest shore of Lake Pukaki were investigated. The sections consisted of the Pukaki Diamicton, which is composed of four lithofacies: (i) homogeneous facies (PDH)—a grey matrix-supported homogeneous subglacial diamicton; (ii) coarse facies (PDC)—a very coarse matrix-supported diamicton, which was interpreted as a proximal glaciolacustrine sediment; (iii) laminated facies (PDL)—a cream coloured, fine-grained, matrix-supported diamicton, with grade laminations of silt, sand and gravel, interpreted as a more distal glaciolacustrine facies; and (iv) fine facies (PDF)—a cream coloured fine-grained, silt-rich matrix-supported diamicton, with lenses of sand and gravels, which was interpreted as the most distal glaciolacustrine facies. It is suggested that these sediments were produced by two small ice advances during a period of general retreat. Furthermore, the sections showed a combination of three types of glaciotectonic deformation; gravity tectonics, proglacial glaciotectonics and subglacial glaciotectonics. Two of the moraines showed an unusual style of glaciotectonic deformation, i.e. proglacial deformation on the proximal face and gravitational slumping on the distal face. It is suggested that this style of deformation is diagnostic of proglacial deformation into a waterbody associated with a retreating margin.